This invention is directed to a blend of a polyarylate and a copolyester which has improved processability relative to the polyarylate and improved weatherability, impact properties and heat distortion temperature relative to the copolyester. The composition of this invention comprises a polyarylate derived from a dihydric phenol and an aromatic dicarboxylic acid, and a copolyester derived from a cyclohexanedimethanol, an alkylene glycol and an aromatic dicarboxylic acid.
Polyarylates are aromatic polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)-propane (also identified as Bisphenol-A) and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids.
Polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They have a high continuous use temperature of about 130.degree. C., and good unnotched toughness, with a pendulum impact value of greater than 300 ft. lbs./in..sup.3. Additionally, polyarylates have inherent flammability and combustion resistance. The polyarylates have good melt stability at high temperatures and good color retention. They also have good processability which allows them to be molded into a variety of articles. However, polyarylates are generally processed by injection molding or extruding at temperatures greater than about 330.degree. C. These high processing temperatures may require the use of special equipment, such as heated molds. Thus, it is desirable to improve the processability of polyarylates.
It has been unexpectedly found that the addition of a copolyester derived from a cyclohexanedimethanol, an alkylene glycol, and an aromatic dicarboxylic acid to a polyarylate improves the processability of the polyarylate in that it increases melt flow resulting in lowering of the molding temperature. Also, molding conditions as well as the mold temperature are lowered by adding the copolyester to the polyarylate.
Polyarylates have good weatherability, i.e. they retain their mechanical properties after exposure to conditions of ultraviolet light and moisture. However, the weatherability of polyesters containing cyclohexanedimethanol groups is generally poor since they loose their mechanical properties, i.e., impact properties upon exposure to ultraviolet light and moisture conditions.
It was thus unexpected that the addition of polyarylate, even in small amounts, to a copolyester containing cyclohexanedimethanol groups, results in a composition having improved weatherability.
Additionally, polymer blends, even those comprised of transparent polymeric materials, usually yield an opaque composition. The blend of polyarylate and copolyester as described herein surprisingly yield transparent compositions.
British Pat. No. 1,002,545 describes a blend wherein one of the components is a polyester of terephthalic acid and one or more aliphatic diols and the other component is a polyarylate of 2,2-di-(4-hydroxyphenyl)-propane and terephthalic and isophthalic acid, the ratio of terephthalate residues to isophthalate residues in the copolyester ranging from 90:10 to 10:90.
The British Patent describes that the blend of the polyester and polyarylate provides an easily moldable polymer blend without the disadvantage when molding each of the two components alone. The examples of the British Patent describe a blend of poly(ethylene terephthalate) and a polyarylate of 2,2-di(4-hydroxyphenyl)propane and isophthalic and terephthalic acid. The British patent states that the aliphatic diol from which the polyester is derived may be cycloaliphatic such as 1,4-di-(hydroxymethyl)cyclohexane.
This British Patent does not suggest that easily moldable blends of a polyarylate of a dihydric phenol and an aromatic dicarboxylic acid can be improved by the addition of a copolyester derived from a cyclohexanedimethanol, an alkylene glycol and a dicarboxylic acid or that the weatherability of the copolyester is improved by the addition of the polyarylate. Also, the British Patent does not suggest that the addition of the copolyester described in this invention to a polyarylate yields better impact strength in a blend with a polyarylate than the blend of polyester and polyarylate described in the British Patent.